Re: [RFC v2] virtio: support packed ring

Michael S. Tsirkin
2018-04-17T12:17:41Z

On Tue, Apr 17, 2018 at 10:51:33AM +0800, Tiwei Bie wrote:
> On Tue, Apr 17, 2018 at 10:11:58AM +0800, Jason Wang wrote:
> > On 2018年04月13日 15:15, Tiwei Bie wrote:
> > > On Fri, Apr 13, 2018 at 12:30:24PM +0800, Jason Wang wrote:
> > > > On 2018年04月01日 22:12, Tiwei Bie wrote:
> [...]
> > > > > +static int detach_buf_packed(struct vring_virtqueue *vq, unsigned int head,
> > > > > + void **ctx)
> > > > > +{
> > > > > + struct vring_packed_desc *desc;
> > > > > + unsigned int i, j;
> > > > > +
> > > > > + /* Clear data ptr. */
> > > > > + vq->desc_state[head].data = NULL;
> > > > > +
> > > > > + i = head;
> > > > > +
> > > > > + for (j = 0; j < vq->desc_state[head].num; j++) {
> > > > > + desc = &vq->vring_packed.desc[i];
> > > > > + vring_unmap_one_packed(vq, desc);
> > > > > + desc->flags = 0x0;
> > > > Looks like this is unnecessary.
> > > It's safer to zero it. If we don't zero it, after we
> > > call virtqueue_detach_unused_buf_packed() which calls
> > > this function, the desc is still available to the
> > > device.
> >
> > Well detach_unused_buf_packed() should be called after device is stopped,
> > otherwise even if you try to clear, there will still be a window that device
> > may use it.
>
> This is not about whether the device has been stopped or
> not. We don't have other places to re-initialize the ring
> descriptors and wrap_counter. So they need to be set to
> the correct values when doing detach_unused_buf.
>
> Best regards,
> Tiwei Bie

find vqs is the time to do it.

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